IRU3033

9

Rev. 1.7

07/17/02

www.irf.com

APPLICATION INFORMATION

Introduction

The IRU3033 device is an application specific product

designed to provide an on board dual supply for the new

generation of microprocessors requiring separate Core

and I/O supplies. One of the processors fitting this re-

quirement is the new Intel P55C

™ multimedia micropro-

cessor. Intel specifies a Core voltage of 2.8V nominal

(

±100mV max) with maximum Core supply current of

6A while the I/O supply is set for 3.3V with a maximum

I/O current of 0.65A. However, in most applications the

I/O regulator also provides the voltage for other IC func-

tions such as the chip set, cache, etc. Typically a low

cost solution such as a Low Dropout (LDO) Linear Regu-

lator is selected to provide the I/O supply with the maxi-

mum designed current of 3A, keeping the power dissi-

pation and the heat sink to a reasonable size. The Core

supply regulator, however, if also selected to be a linear

regulator, will be dissipating a maximum of 12.6W [(5V-

2.8V)X5.7A] of power, which requires a substantial

amount of heat sinking and perhaps forced air cooling in

order to keep it operational. Some manufacturers sug-

gest using two regulators to current share and therefore

distribute the power dissipation equally between the regu-

lators. The problem is that, in order to equally current

share you need to sense both currents and force the

slave regulator to match the master regulator. This can

be done, but at the cost of the circuit complexity and

much higher system cost and the total power dissipa-

tion is still the same. In fact, if the task is to design a

flexible motherboard to accommodate the Cyrix 6X86L

or their future MMX processors, then the power dissipa-

tion could easily reach 20W or more. At this power dis-

sipation level the choice for a switching regulator ap-

proach becomes evident. However, the main reason that

designers have always shied away from the switching

regulators is their higher price tag and more complex

circuit design that is associated with this kind of tech-

nique.

The IRU3033 device is designed to take advantage of

the high efficiency of the switching regulator technique

for the Core supply while maintaining the low cost LDO

regulator for the I/O supply by offering both control func-

tions in a single 8-pin surface mount package. In fact,

as the typical application circuit shows, one can design

a complete flexible motherboard using the IRU3033 and

a few external components yielding a very low compo-

nent count switching regulator and with an addition of a

low cost pass transistor for the I/O supply provide a com-

plete dual supply power solution.

LDO Section

The output voltage of the LDO regulator is externally pro-

grammable via 2 external resistors from 1.25V to 5V.

The internal voltage reference of the LDO regulator is set

to 1.25V and the output of the regulator can be pro-

grammed using the following formula:

Where:

IRU3033.

The IRU3033 requires the use of an output capacitor as

part of the frequency compensation in order to be stable.

Typical designs for the microprocessor applications use

standard electrolytic capacitors with typical ESR in the

range of 50 to 100m

V and an output capacitance of 500

to 1000

mF. Fortunately as the capacitance increases,

the ESR decreases resulting in a fixed RC time con-

stant. The IRU3033 takes advantage of this phenomena

in making the overall regulator loop stable. For most ap-

plications a minimum of 100

mF aluminum electrolytic

capacitor such as Sanyo, MVGX series, Panasonic FA

series or Nichicon PL series insures both stability and

good transient response.

An external filtering is suggested as shown in the appli-

cation circuit that reduces the switching ripple that might

show in the output of the LDO regulator.

Switching Controller Operation

The operation of the switching controller is as follows:

After the power is applied, the output drive (Drv1) goes to

100% duty cycle and the current in the inductor charges

the output capacitor causing the output voltage to in-

crease. When output reaches a pre-programmed set

high which jumps the feedback pin higher than 1.25V

resulting in a fixed output ripple which is given by the

following equation:

Where:

Rt = Top resistor of the output divider, resistor con-